core: add key alt+backspace, change behavior of key ctrl+w (closes #559)

The key ctrl+w now deletes one word until whitespace.

The new key alt+backspace deletes one word (same behavior as ctrl+w in previous
releases).

src/core/wee-string.c

@@ -1109,6 +1109,20 @@ string_convert_escaped_chars (const char *string)
     return output;
 }
 
+/*
+ * Checks if first char of string is a whitespace (space or tab).
+ *
+ * Returns:
+ *   1: first char is whitespace
+ *   0: first char is not whitespace
+ */
+
+int
+string_is_whitespace_char (const char *string)
+{
+    return (string && ((string[0] == ' ') || string[0] == '\t')) ? 1 : 0;
+}
+
 /*
  * Checks if first char of string is a "word char".
  *